S/PDIF output

From HwB

S/PDIF output buffer circuits

Here is set of output circuit which take in TTL level S/PDIF signal in and output standard S/PDIF coax cable output signal. Those output circuit are very useful building blocks for all kinds of S/PDIF related electronics projects. All the circuits do basically the same thing. They just use different components, so use the one where you can easily get components.

The transformer used in this circuit is 1:1 pulse transformer (the original application note suggested pulse transformer from Pulse Engineering or Schott Corporation). IC1 is a buffer IC which must be capable to supply enough current to drive the 75 Ω S/PDIF interface to full voltage swing specified (capable of loading 415 Ω load or capable of supplying 12 mA current).

This circuit does not provide any isolation on the output. The TTL level signal form 74HCT04 buffer is attenuated to the levels specified in S/PDIF optical interface by using the voltage divider made of 374 Ω and 93.1 Ω resistors. This circuit is based on S/PDIF converter circuit diagram from Elektor Electronics magazine issue 4/1997 page 66.

Simplest TTL to S/PDIF coax interface

This circuit interfaces directly to the TTL level output provided by some sound equipment (CD-ROM or sound card). This circuit works if the equipment which gives out TTL level has powerful enough TTL output to run this circuit (enough current output capability). If this simple circuit does not work reliably, then you need to build a circuit which includes the buffering electronics.

Isolation transformer for S/PDIF output

If your equipment (for example sound cards) has an S/PDIF output which does not have an isolation transformer at the output, you can easily add the transformer to it by putting a suitable transformer afther the original output.

Elector Electronics magazine issue 7-8/1999 recommend building the transformer for S/PDIF isolation in the following way:

The transformer must have good coupling factor, so the transformer core must be a toroidal core made of high permiability material. The prototype described in the magazine uses Philips Type TN13/7.5/5-3E25 core which has permeability rating (yt) of 4500. The primary and secondary windings consisted of 6 turns of 0.5 mm diameter enamelled copper wire laid on opposite sides of toroid. The transformer used in this prototype was described to have a bandwidth raged from 50 kHz to 17 MHz, which is more than adequate for an S/PDIF link.

The transformer should be fitted directly to the source and the receiving end must be properly terminated for reliable operation. The reason for this is that the transformer input and output impedances are not exactly 75 Ω.